This invention relates to a position detecting device and, more particularly, to a position detecting device usable with a semiconductor exposure apparatus, for detecting at a high accuracy the position of each of alignment marks formed on a mask and a wafer to permit accurate alignment between the mask and wafer prior to tranferring an integrated circuit pattern of the mask onto the wafer. The invention also relates to an alignment system including a position detecting device as above for aligning a mask with a wafer. Hereinafter, the alignment mark formed on the mask will be referred to as "mask alignment mark" and the alignment mark formed on a wafer will be referred to as "wafer alignment mark". Also, in this specification, the word "mask" is comprehensive of a reticle as well as a mask.
Many types of conventional exposure apparatuses for use in the manufacture of semiconductor circuit devices employ arrangement for detecting, through laser beam scanning, the positions of the mask alignment mark and wafer alignment mark in order to achieve alignment between the mask and wafer. According to one of such position detecting techniques, the mask and wafer which have been coarsely aligned with each other are optically scanned and then the light beam scatteringly reflected from the alignment marks is detected by a photodetector in a time series manner, whereby the positional information on the marks is converted into the information with respect to time.
In such case, however, the light beams scatteringly reflected by the wafer alignment mark are liable to be affected by a photoresist layer applied to the surface of the wafer, whereby the accuracy of position detecting is disadvantageously deteriorated. Such deterioration of detecting accuracy is chiefly because of the prism action of the photoresist layer, as is known in the art.